Heretofore, as a flush water supply device for supplying flush water to a flush water tank of a flush toilet, there has been known one type which comprises a small tank formed in a tub-like shape having a bottom formed with an opening, and a float provided inside the small tank, and a water supply valve which is an openable/closable value adapted to be moved to supply and stop flush water with respect to the flush water tank interlockingly with an up-down movement of the float, as described, for example, Patent Document 1 (JP 3655318B). The water supply valve of the flush water supply device is constructed as a so-called “diaphragm” type which comprises a diaphragm valve provided on an upper side of a water supply pipe and a vacuum break valve integrally provided in the diaphragm valve. The flush water supply device further comprises a filer member provided in an intermediate passage of the water supply pipe on an upstream side of the water supply valve.